This worked like a dream. There appears to be one issue, though. On OS update, it appears that the EFI/BOOT/grub.cfg is overwritten.
My BOOT_ORDER was set to B A. I applied this fix, and it worked, booting to Slot B. After an OS update, it started booting to Slot A. I thought that everything was working as it should. Then, in the next update, it kept booting to Slot A. I inspected the EFI/BOOT/grub.cfg and the updates were gone.
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